Material handling machines are available in a wide variety of configurations to suit particular applications. Such machines include fork-type lift trucks, front end loaders and many others.
Another type of material handling machine (and the one to which the invention relates), is known as an overhead travelling crane. In a common configuration, such cranes include a pair of bridge girders spanning rather widely spaced railroad type bridge rails. Such rails are suspended above, for example, a factory floor or an outdoor steel handling yard. The girders are supported and propelled by flanged wheels riding atop the rails.
Mounted atop each girder and extending along its length is a trolley rail, atop which is mounted a trolley capable of "traversing" movement, i.e., movement along a line generally normal to the line of movement of the entire crane. The trolley is equipped with at least one hoist drive and a load-hoisting hook (or other load-handling device) suspended from a bottom block for moving loads from place to place. So configured, the crane is capable of lifting a load from any location on a factory floor, for example, and moving it to any other location.
A factor considered by designers of overhead travelling cranes is the possibility of the bottom block being raised to an elevation at which it strikes the solid undercarriage of the trolley. In that event, there is a substantial risk of breaking the stranded rope-like steel cables by which the bottom block is attached to the rotating hoist drum. If a cable breaks, there is a chance that the load will be uncontrollably dropped.
To help guard against that eventuality, crane designers have employed a control circuit limit switch and a power circuit limit switch actuated in one of the ways described below. If the bottom block reaches a certain elevation, the control limit switch is tripped. Such limit switch tripping disables the control circuit or, in the alternative, "reconfigures" the control circuit in such a way that the hoist drive controller causes the rate of bottom block ascent to slow markedly. If the operator fails to stop the hoist drive or if the control limit switch malfunctions for some reason, the bottom block continues its upward movement and trips the power limit switch which opens the power connections to the hoist drive motor and stops bottom block movement before such block strikes the trolley undercarriage.
A typical known arrangement includes a control limit switch coupled to the rotating hoist drum shaft or to the shaft of an intermediate gear reduction installed between the hoist motor and the hoist drum. The power circuit limit switch has a heavy block-shaped weight suspended from one end of a counterweighted arm. The torque produced by the weight is greater than that produced by the counterweight and the weight retains the power switch in the operative position.
On the other hand, when the weight is lifted by the ascending bottom block, the counterweight "takes over" and trips the power switch. Viewed another way, the control circuit limit switch is actuated by rotary shaft motion and the power limit switch is actuated by the upward linear motion of the bottom block.
In another somewhat less common arrangement involving two hoists (and two bottom blocks) on a single hoist drive, the control circuit limit switch is also equipped with a suspended weight. Such switch is tripped when one of the bottom blocks lifts the weight; the power circuit limit switch is tripped when the other bottom block lifts that limit switch weight.
While these arrangements have been generally satisfactory for their intended purpose, they are attended by certain disadvantages. For example, if it is desired to use both a power limit switch and a control limit switch of the weighted type, it has been necessary to use two bottom blocks, one for each switch.
Yet another disadvantage is that the block-like weight suspended from a limit switch represents a relatively small "target" to be contacted and lifted by the bottom block. Notwithstanding precautionary measures, e.g., a bail-like structure looped around the dead ends of the hoisting cable to act as a guide, there have been instances where the bottom block fails to contact the limit switch weight. Broken hoist cable can result.
A limit switch weight apparatus useful in cranes and overcoming the aforementioned disadvantages would be an important advance in the art.